Plasma Amination of Low‐Density Polyethylene by DBD Afterglows at Atmospheric Pressure

Abstract

Surfaces of LDPE can be functionalized with primary amino groups using the afterglows from dielectric barrier discharges in N2, N2 + H2, and N2 + NH3 mixtures at atmospheric pressure. Quantitative analysis by a recently developed ATR FT‐IR spectroscopic method allows calculation of the number of NH2 groups generated per unit area. Treatment in N2 + H2 afterglows was found to be most efficient in terms of area densities of primary amines achieved (more than 15 nm−2), but appreciable quantities of NH2, up to about 3 nm−2, are also obtained in pure N2. Possible mechanisms of the amination reaction and the roles of gas additives are discussed.

Density and Aging Behavior of Primary Amino Groups on Afterglow Plasma‐Treated Low‐Density Polyethylene (LDPE)

“…In both gas mixtures, the amino density increases over this period continuously without saturation. While up to 5 and 15 amino groups per square nanometer were obtained in similar previous experiments utilizing pure N 2 and N 2 þ 2.5% H 2 mixtures, [9] the maximum attainable amino densities after prolonged treatment here stay well below 1 nm À2 and …”

“…Investigations with increased delay times (10 and 29 ms), obtained at positions further downstream from the discharge, showed for both gas mixtures, Ar þ 1.7% NH 3 and N 2 þ 1.7% NH 3 , a strongly decreased incorporation of amino groups after 45 s treatment time, different from what was earlier obtained during investigations in pure nitrogen, where the amination declined only slowly with increased delay time. Similar to the effect of hydrogen addition on the XPS spectrum of plasmaaminated LDPE, reported earlier, [9] also ammonia additions lead to a reduced intensity of C1s photoelectrons with binding energies near 288 eV, indicating a reduced incorporation of C¼O containing groups like amides. At this opportunity, we would like to give a short comment on the question whether a similar mechanism of plasma amination as proposed in the DBD afterglow, namely, an attack at the polymer surface by N 2 (A) and subsequent reaction with N( 4 S), might also be the prevailing plasma-amination mechanism within the discharge if polymers are directly contacted with the DBD.…”

“…[9,15] However, more investigations are required to come to a sound conclusion about chemical processes proceeding on the time scale of seconds and hours after the exposure of the freshly plasma-treated surface to humid laboratory air. After 6 weeks, the amount of primary amino groups at the surface (according to XPS measurements of the [NH 2 ]/ [C] ratio) has reached the same level for all gas mixtures.…”

“…For the experiments, a tubular flow reactor was used as it is illustrated in the previous publication. [6,9] Gas flows of 10 L Á min À1 STP were used, corresponding to an afterglow delay time of 6 ms at the normally used position of the polymer foil. The treatment time for the investigation of the aging behavior was 45 s. The discharge was powered by a generator 7010 R from SOFTAL Electronics GmbH.…”

“…In our view, these experiments again illustrate the important role played by the excited nitrogen molecule in its lowest triplet state N 2 (A), which is much more reactive to aliphatic hydrocarbon molecules, and might be considered as model compounds for polyolefins, than the ground state atom N( 4 S) or hydrogen atoms and hydrogen-bearing radicals NH and NH 2 , see the literature cited in our previous publication. [9] In case of the Ar þ 1.7% NH 3 mixture, only small concentrations of nitrogen are available in the gas phase; the probability of formation of N 2 (A) by the recombination of N atoms in the vicinity of the polymer surface is presumably quite small. In addition, N 2 (A) reacts very rapidly with NH 3 , dissociating it into H and NH 2 , [13] which themselves are much less reactive toward alkanes than N 2 (A).…”